The present invention is directed to a drill bit with a generally axially extending cylindrically shaped shank for use in hand-held drilling machines.
Known hand-held drilling machines used to cut bores in a hard surface material, such as concrete, rock and the like, can be divided into three groups, that is, drilling machines, percussion drilling machines and hammer drills.
Plain drilling machines are of only secondary importance at the present time and are simple hand tools which merely impart a rotary motion to an inserted drill bit. In such drilling machines, a clamping chuck is used for receiving the drill bit with the generally cylindrically shaped shank of the bit clamped in the chuck.
Percussion drills also use a clamping chuck in which the drill bits used in drilling machines can also be clamped. The difference between a percussion drill and a plain drilling machine is that in a percussion drill the chuck is positioned on an axially displaceable spindle which experiences a reciprocating movement during the drilling operation. The reciprocating movement is developed in the drilling tool by ratchet discs and is transmitted by the chuck to the drill bit tightly clamped in it.
At the present time, hammer drills experience the greatest use in drilling hard surface materials. Such drilling devices also have a chuck, however, unlike plain drilling machines and percussion drilling machines, the drill bit is held so that it is movable axially to a limited extent while it is positively locked for the transmission of rotational movement. By elements located in the hammer drill, such as an electro-pneumatic percussion mechanism, a piston is reciprocated back and forth for imparting a percussion force to the drill bit either directly or by the interposition of another piston. Because of the axial displaceability with respect to the chuck, no percussive force is transmitted to the chuck and the entire percussive energy is available for the drilling operation.
To positively lock and clamp drill bits in hammer drills, it is conventional to form the drill bits with a standard shank containing entrainment faces or surfaces for radially displaceable locking elements in the chuck. Since such a standard shank has resulted in considerable cost increases for the drill bit, recently variable shanks have been developed. Such shanks are clamped in the chucks by means of entrainment surfaces in the shank with the chucks displaying a variable guidance area and counter surfaces for the entrainment surfaces formed in the drill bits.
Since hammer drills have considerable advantages in use as compared with percussion drilling machines, and because the development of variable shanks has rendered the drill bits more favorable from an economic point of view, hammer drills have, to a considerable extent, replaced percussion drilling machines. As a result, an organization using drilling devices often has both percussion drilling machines as well as hammer drills. Accordingly, based on the present trend, as a rule, only drill bits for hammer drills will be purchased so that there is the requirement that such drill bits can also be used in percussion drilling machines.
For solving this problem a drill bit is known having a shank containing grooves for forming the entrainment surfaces arranged in such a manner that the clamping jaws of a chuck can not engage in the grooves, if such a drill bit is inserted into a percussion drill. As a result, the same situation exists as when a drill bit with a cylindrical shank is clamped in a percussion drill, since because of the pure frictional lock that is required there must be a considerable clamping force exerted by the chuck. Since only reciprocating movement acts axially on the drill bit, the forces exerted in the axial direction are of no significance, rather the significant forces are those required for the transmission of torque. Accordingly, if insufficient clamping forces are exerted by the chuck, there is the danger that the requisite torque is not completely transmitted to the drill bit and the drill bit may turn relative to the chuck. While such a drill bit may be used in all types of drilling devices, it does not provide any improvement over the conventional tool if it is employed in percussion drilling machines.
To improve drill bits for use in different types of drilling machines another known solution has provided the drill bit shank with a relatively long groove closed at the trailing end of the bit for forming entrainment surfaces. While the use of such a drill bit in a hammer drill results in the same situation as in the previously discussed drill bit, it is expected that with this arrangement of the groove an improvement for use in percussion drilling machines can be obtained. In a limited number of chucks, that is, in chucks where the rear end of the clamping jaws are spaced a sufficient distance from the abutment face on the chuck side for the trailing end face of the drill bits, an advantage is achieved in that the clamping jaws can extend completely into the grooves in the shank of the drill bit. In a large number of clamping chucks, presently on the market, such a distance or spacing is not present and the clamping jaws can not extend into the grooves, because of the closed end of the grooves required for use in hammer drills.
Apart from the disadvantage that there is insufficient rotational entrainment, there is another disadvantage in that the drill bit is in contact with the clamping jaws along a relatively short distance so that a ball joint is created which causes the drill bit to turn out of round by a considerable amount.